As a system for reducing the steering effort required by a vehicle operator, or a so-called power steering system, the type, for instance, disclosed in Japanese patent publication (kokoku) No. 50-33584 is known. According to this power steering system, the steering torque applied from a steering wheel is assisted by an output torque of an electric motor. By varying the gain for the detected signal corresponding to the steering torque applied to the steering wheel by a vehicle operator according to the vehicle speed and the road condition, the output torque of the assisting electric motor is adjusted so that an optimum steering torque may be obtained at all times.
When a vehicle encounters a strong crosswind or side wind, the vehicle tends to deviate from the intended straight path of travel. To maintain the vehicle on the straight path, it is necessary to apply a reaction to the steerable wheels which opposes such an external disturbance.
According to the conventional power steering system, however, the assisting torque is produced only when the vehicle operator has applied some steering torque to the steering wheel. Therefore, when the vehicle deviates from the intended straight path of travel due to crosswind, the electric motor will not produce any assisting torque by itself.
Therefore, to prevent the deviation of the vehicle from the intended path, the vehicle operator must constantly apply a manual steering torque to the steering wheel. According to the conventional steering system, the assisting torque is generally small when the lateral acceleration and the yaw rate of the vehicle are large. Therefore, when the vehicle deviates from an intended path of travel, the steering input necessary to restore the vehicle to the intended path of travel becomes progressively greater as the lateral acceleration and the yaw rate of the vehicle increase.
Thus, to overcome this inconvenience, it has been proposed to detect an irregular behavior of the vehicle caused by an external disturbance from the yaw rate or lateral acceleration of the vehicle, and to produce a reaction with an electric motor for producing a steering torque which cancels the effect of such a disturbance so that the irregular behavior of the vehicle due to external disturbances may be controlled.
According to such a control system using an electric motor, the steering output was given as a combination of a yaw rate reaction component for stabilizing the lateral movement of the vehicle and a damping component for stabilizing the movement of the steering wheel. The damping component for stabilizing the movement of the steering wheel was simply given as a product of a steering wheel angular speed and a constant damping coefficient.
However, for instance, when the yaw rate reaction coefficient is increased so as to ensure a sufficient control over the lateral movement of the vehicle in case of a strong crosswind, the damping coefficient for the movement of the steering wheel must be also increased so that the oscillatory movement of the steering wheel may be quickly attenuated. As a result, the vehicle operator will encounter a strong resistance when he tries to abruptly turn the steering wheel, and not only the vehicle operator experiences an unfamiliar feeling, but also the operability of the steering wheel is impaired. On the other hand, simply reducing the damping coefficient will reduce the rate of attenuation of the oscillatory angular movement of the steering wheel to an unacceptable level.